1. Field of the Invention
The present invention relates to a magnetic recording medium and an information recording apparatus configured to record information using a function of assisting magnetization switching of magnetic grains by irradiating a magnetic recording medium with a high-frequency magnetic field (hereinafter this may be referred to as micro wave).
2. Background Art
For higher recording density of magnetic recording, a recording bit size has to be reduced. Then, in order to stably maintain the magnetization of small recording bits, the recording medium has to be made of a magnetic material having high anisotropy field (or with high magnetic coercive force). Recording on such a magnetic recording medium having high anisotropy field will require intense recording magnetic field corresponding thereto. Since it is difficult to intensify the magnetic field for recording by a recording head, recording may fail due to the shortage of the magnetic field. To solve such a problem, methods such as microwave assisted magnetic recording and thermally assisted magnetic recording have been studied. These are methods of temporarily reducing the anisotropy field of the magnetic recording medium by irradiating it with high-frequency magnetic field or heat, thus enabling recording on a magnetic recording medium having high anisotropy field.
For microwave assisted magnetic recording, a magnetic recording medium was irradiated with high-frequency magnetic field of a microwave band, thus exciting precession of spin in magnetic material by using magnetic resonance effect and reducing magnetization switching field, and records information on the magnetic recording medium. For magnetic resonance, the temperature of the magnetic recording medium does not rise probably since selected spins are excited although microwaves at a frequency proportional to the anisotropy field of the magnetic recording medium have to be used.
U.S. Pat. No. 7,616,412 discloses a technique for microwave assisted recording, and according to this technique, a magnetized rotating body rotating rapidly due to spin torque is disposed for generation of high-frequency magnetic field in the vicinity of a magnetic recording medium that is adjacent to a main pole of a perpendicular magnetic head. WO2009/133786 discloses a technique for a microwave assisted magnetic recording medium, including a magnetic (resonance) layer on a surface side of the magnetic recording medium and a recording layer on a substrate side, the resonance layer being induced by high-frequency magnetic field and the recording layer having larger anisotropy field and being switched following the resonance layer.
For higher recording density, the size of recording bits has to be made smaller, and in order to record information on such small recording bits precisely, the distribution of magnetic field (magnetic switching field) that is necessary for magnetization switching of magnetic grains making up the magnetic recording medium has to be made smaller. In the case of large switching field distribution, not only specific intended magnetic grains but also a part of surrounding magnetic gains will be switched by the recording magnetic field of a head. A known method to reduce the switching field distribution is to introduce suitable exchange coupling between neighboring magnetic grains. For instance, IEEE TRANSACTIONS ON MAGNETICS, vol. 43, No. 6, 2142 (2007) describes suppressed switching field distribution by introducing exchange coupling between dots of a bit-patterned medium.
As a result of further study to reduce switching field distribution for microwave assisted recording, however, it was found that the exchange coupling given between magnetic grains of the magnetic recording medium often causes a failure to record information precisely on small recording bits in the microwave assisted recording. Even in case of the small switching field distribution in terms of static magnetic properties, this is not always reflected in the recording characteristics of microwave assisted recording. It is necessary for microwave assisted recording to reduce the switching field by applying high-frequency magnetic field, which is different from conventional recording methods. Thus, in order to realize high density recording by recording information on small recording bits in microwave assisted recording, both of reduction in switching field distribution and reduction in switching field intensity by high-frequency magnetic field have to be achieved.